This invention relates to the art of polybenzazole (PBZ) polymers and processes for synthesizing them.
Polybenzazole polymers are a known class of polymers comprising polybenzoxazole (PBO), polybenzothiazole (PBT), polybenzimidazole (PBI) and copolymers of these polymers. Polybenzazoles are generally classified as AB-polymers, containing a plurality of mer units depicted in formula 1(a), or AA/BB, containing a plurality of mer units which conform with formula 1(b) ##STR1## wherein: each Ar is an aromatic group,
DM is a divalent organic moiety which is stable and inert in acid under polymerization conditions, and PA0 each Z is independently an oxygen atom, a sulfur atom, or a nitrogen atom that is bonded to a hydrogen atom or an organic moiety which is stable and inert in acid under polymerization conditions. PA0 (1) each nitrogen atom and Z group within a given azole ring are bonded to the aromatic group in ortho position with respect to each other; and PA0 (2) if the mer unit has two azole rings, one nitrogen atom and Z moiety may be in either cis position or trans position with respect to the other nitrogen atom and Z moiety, for example illustrated in 11 Ency. Poly. Sci. & Eng., supra, at 602, which is incorporated herein by reference. The same understandings apply with respect to amine groups and Z moieties in a BB-PBZ monomer.)
(For the purpose of this application, when the nitrogen atoms and Z moieties of a mer unit are depicted as bonded to an aromatic group without indicating their position, as in Formulae 1(a)-(b), it shall be understood that:
Polybenzazoles are synthesized by the reaction of one or more difunctional monomers, each of which contains at least two azole-forming moieties chosen from the group consisting of electron-deficient carbon groups and o-amino-basic moieties. Such reactions are illustrated in Formulae 2(a) and (b). ##STR2## wherein each Q is an electron-deficient carbon group and all other moieties have the meaning and preferred embodiments previously given. The monomer shown in Formula 2(a) is ordinarily referred to as an AB-monomer, and the monomers in FIG. 2(b) are ordinarily called BB- and AA-monomers. The divalent organic moiety DM of the AA-monomer ordinarily comprises an aromatic group, which is most usually a p-phenylene group. The reaction is ordinarily carried out under non-oxidizing conditions in a non-oxidizing, dehydrating solvent acid, which is most frequently polyphosphoric acid.
Polybenzazole polymers, their properties and their synthesis are discussed in detail in the following references: Sybert et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,772,678 (Sep. 20, 1988); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,692 (Aug. 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat. No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985); Imai et al. "Polybenzoxazoles and Polybenzothiazoles," 83 Makromol. Chem. 167 (1965), Evers, Thermooxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (Mar. 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988), which are incorporated herein by reference.
The o-amino-basic groups in AB- and BB-PBZ monomers are susceptible to oxidation. This is particularly true for BB-PBZ monomers of PBO and PBT. The oxidation problem can be partially avoided by storing the monomers as hydrogen chloride salts, but even acid salts experience some oxidation over time. Furthermore, the monomer can become oxidized by thermal degradation during polymerization at too high a temperature.
Oxidation may upset stoichiometry in polymerization and hold down the molecular weight of the resulting polymer. It may also cause the resulting polymer to have an undesirably dark color. It is desirable to minimize the effects of oxidized AB- and BB-PBZ monomers on polymerization.